In several fields and especially in the field of pharmaceutical compositions there is a need for compositions having controlled release of active ingredients. In the case of pharmaceutical compositions such units are of special importance when a long lasting steady blood level of the active ingredient administered to the body is required to obtain a suitable therapeutic effect. These active ingredients are e.g. analgesics, pharmaceutical compositions acting upon the circulatory and vascular system, sedatives, and antihistamines. The release of the active ingredient is ensured for a longer time by administering additional doses of the composition containing the active ingredient at certain intervals, due to which, however, a fluctuation of the blood level takes place. This disadvantage should be eliminated by compositions for controlled release of active ingredients.
Several methods are known for the preparation of compositions for controlled release of active ingredients. In some methods the active ingredient is admixed with inert substances such as waxes or polymers and the mixture is compressed into tablets. The preparation of such so called frame tablets is described in the German patent publication No. 1 258 458. A relatively large amount of inert frame material is required to prepare such tablets and thus voluminous and not easily deglutible tablets are obtained. In many cases a very high temperature is required to compress the tablets and the active ingredient may be damaged by such temperatures.
The preparation of compositions of controlled release of active ingredients has been improved by introducing varnishes, which ensure the delayed release of the active ingredients by diffusion. Various homo and copolymers are used to prepare varnishes of suitable diffusion rates (see e.g. British patent specification No. 833 458 and German Pat. Nos. 1 279 896 and 1 949 894). The required mechanical properties and the suitable tenacity of the varnish layer are obtained by emollients and other additives. The addition of emollients is especially important if the basic material of the polymer forming the varnish is a solid vitreous substance at room temperature, such as polymers of the acrylate and metacrylate type. On storage of the tablets, however, the emollients and the various additives easily diffuse and migrate from the thin varnish film layers. Thus the varnish materials of the tablets become brittle on storing, and the permeability properties change. To avoid this in some cases a relatively larger amount of emollient is employed. The employment of a large amount of emollients is however disadvantageous in other respects. An important part of the emollients consists of an ester type or polyalcohol type small molecule and as such they are a good medium for the growth of microorganisms. That is why the use of emollient-containing plastics as packing material in the food industry is limited. Similarly the use of such substances in a protecting covering of pharmaceutical compositions should be avoided. It is difficult to control the release of active ingredients by varnishes if the permeability of the varnish has to be adjusted for active ingredient molecules of definite size and diffusion. It is rather complicated to prepare a varnish forming polymer for each active ingredient molecule; thus the desired varnish is usually prepared by mixing together a varnish of good permeability and a varnish of low permeability in a suitable ratio.
It is known however, that various polymers even copolymers of identical composition but of different monomer ratio, are generally not well miscible. After the removal of the solvent such polymer mixtures often separate and a product of poor quality is obtained.
Similarly problems arise if plant protecting agents, insecticides, pheromones or other biologically active ingredients with delayed action need to be prepared.